Cooling electronic devices in a data center

ABSTRACT

A server rack thermosiphon system includes a plurality of evaporators, each evaporator including a thermal interface for one or more heat-generating server rack devices; at least one condenser mounted to an external structure of a server rack, the condenser including a fluid-cooled heat transfer module; a liquid conduit that fluidly couples each of the evaporators to the condenser to deliver a liquid phase of a working fluid from the condenser to the evaporators; and a vapor conduit that fluidly couples each of the evaporators to the condenser to deliver a mixed phase of the working fluid from the evaporators to the condenser.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of, and claims priority under 35U.S.C. § 120 to, U.S. patent application Ser. No. 14/703,566, filed onMay 4, 2015, the entire contents of the previous application isincorporated by reference herein.

TECHNICAL FIELD

This disclosure relates to systems and methods for providing cooling toelectronic equipment, such as computer server racks and relatedequipment in computer data centers, with a thermosiphon system.

BACKGROUND

Computer users often focus on the speed of computer microprocessors(e.g., megahertz and gigahertz). Many forget that this speed often comeswith a cost—higher power consumption. This power consumption alsogenerates heat. That is because, by simple laws of physics, all thepower has to go somewhere, and that somewhere is, in the end, conversioninto heat. A pair of microprocessors mounted on a single motherboard candraw hundreds of watts or more of power. Multiply that figure by severalthousand (or tens of thousands) to account for the many computers in alarge data center, and one can readily appreciate the amount of heatthat can be generated. The effects of power consumed by the criticalload in the data center are often compounded when one incorporates allof the ancillary equipment required to support the critical load.

Many techniques may be used to cool electronic devices (e.g.,processors, memories, networking devices, and other heat-generatingdevices) that are located on a server or network rack tray. Forinstance, forced convection may be created by providing a coolingairflow over the devices. Fans located near the devices, fans located incomputer server rooms, and/or fans located in ductwork in fluidcommunication with the air surrounding the electronic devices, may forcethe cooling airflow over the tray containing the devices. In someinstances, one or more components or devices on a server tray may belocated in a difficult-to-cool area of the tray; for example, an areawhere forced convection is not particularly effective or not available.

The consequence of inadequate and/or insufficient cooling may be thefailure of one or more electronic devices on the tray due to atemperature of the device exceeding a maximum rated temperature. Whilecertain redundancies may be built into a computer data center, a serverrack, and even individual trays, the failure of devices due tooverheating can come at a great cost in terms of speed, efficiency, andexpense.

Thermosiphons are heat exchangers that operate using a fluid thatundergoes a phase change. A liquid form of the fluid is vaporized in anevaporator, and heat is carried by the vapor form of the fluid from theevaporator to a condenser. In the condenser, the vapor condenses, andthe liquid form of the fluid is then returned via gravity to theevaporator. Thus, the fluid circulates between the evaporator and thecondenser without need of a mechanical pump.

SUMMARY

This disclosure describes implementations of a thermosiphon system thatcools electronic heat-generating devices mounted in a server rack of adata center. The thermosiphon system includes multiple evaporatormodules that are in thermal contact with, or may be placed into thermalcontact with, the heat-generating devices. When in thermal contact,solid surface components of the evaporator modules and heat-generatingdevices, respectively, may be placed into physical contact (through athermal interface material or otherwise) to create a thermal interfacethrough which heat flows from the heat-generating devices to the workingfluid (e.g., liquid phase) circulated in the evaporator modules. Aliquid phase of a working fluid is circulated (e.g., naturally) throughthe evaporator modules, into which heat from the heat-generating devicesis transferred. As the heat is transferred to the working fluid, theliquid phase changes to a vapor phase or a mixed vapor-liquid phase(e.g., depending on the amount of transferred heat). The vapor, ormixed, phase circulates to a condenser module of the thermosiphonsystem, where it is changed back to the liquid phase by a cooling fluidthat is circulated through the condenser module. In some aspects, thecondenser module may be mounted to the server rack, such as to a topsurface of the server rack. In some aspects, there may be a singlecondenser module fluidly coupled to the multiple evaporator modules. Insome aspects, a ratio of condenser modules to evaporator modules in thethermosiphon system may be less than one.

In an example implementation, a data center thermosiphon cooling systemincludes a plurality of evaporator modules, each evaporator moduleconfigured to thermally couple to one or more heat-generating devicesmountable in a rack of a data center; a condenser module that includes aheat transfer surface and is mounted external to the rack; and aplurality of transport members. Each transport member includes a liquidconduit that fluidly couples an inlet of a respective evaporator moduleto deliver a liquid phase of a working fluid from the condenser to therespective evaporator; and a vapor conduit that fluidly couples anoutlet of the respective evaporator module to the condenser to deliver amixed-phase of the working fluid from the respective evaporator to thecondenser, the mixed-phase of the working fluid including heattransferred from the one or more heat-generating devices to the workingfluid through the respective evaporator.

In a first aspect combinable with the example implementation, thecondenser module is mounted on a top surface of the rack.

A second aspect combinable with any of the previous aspects furtherincludes a liquid header fluidly coupled to each of the liquid conduitsand to an outlet of the condenser module.

A third aspect combinable with any of the previous aspects furtherincludes a vapor header fluidly coupled to each of the vapor conduitsand an inlet of the condenser module.

In a fourth aspect combinable with any of the previous aspects, theliquid and vapor headers are mounted external to the rack.

A fifth aspect combinable with any of the previous aspects furtherincludes a respective fluid disconnect that couples each liquid conduitto the liquid header.

In a sixth aspect combinable with any of the previous aspects, eachrespective fluid disconnect is configured to decouple the respectiveevaporator module from the condenser module and seal the liquid phase ofthe working fluid in at least one of the respective evaporator module orthe respective liquid conduit.

In a seventh aspect combinable with any of the previous aspects, thefluid disconnect includes an orifice configured to meter an amount ofthe liquid phase of the working fluid to the respective evaporator.

In an eighth aspect combinable with any of the previous aspects, thecondenser module includes an air-cooled condenser.

In a ninth aspect combinable with any of the previous aspects, theair-cooled condenser includes one or more fans positioned to circulate acooling airflow over the heat transfer surface.

In a tenth aspect combinable with any of the previous aspects, thecondenser module includes a liquid-cooled condenser including a coolingliquid inlet and a cooling liquid outlet.

In an eleventh aspect combinable with any of the previous aspects, thecondenser module includes a single condenser heat transfer coil.

A twelfth aspect combinable with any of the previous aspects furtherincludes a plurality of tray positioners.

In a thirteenth aspect combinable with any of the previous aspects, eachtray positioner configured to urge one or more heat-generating devicessupported on a respective tray assembly and a respective evaporatormodule into thermal contact.

In a fourteenth aspect combinable with any of the previous aspects, eachrespective tray assembly includes a vertically-mountable tray assembly.

In a fifteenth aspect combinable with any of the previous aspects, eachtray positioner includes a cam assembly.

In another example implementation, a method for cooling heat-generatingelectronic devices in a data center includes flowing a liquid phase of aworking fluid from a condenser module of a thermosiphon system that ismounted external to a server rack in a data center to a plurality oftransport members of the thermosiphon system; flowing the liquid phaseof the working fluid from the plurality of transport members to aplurality of evaporator modules of the thermosiphon system, each of theevaporator modules thermally coupled to one or more heat-generatingdevices mounted in the inner volume of the rack; receiving heat from theone or more heat-generating devices in the liquid phase of the workingfluid to boil a portion of the liquid phase of the working fluid; andflowing a mixed phase of the working fluid from the plurality ofevaporator modules, through the plurality of transport members, to thecondenser module.

A first aspect combinable with the example implementation furtherincludes flowing the mixed phase of the working fluid to the condensermodule mounted on a top surface of the server rack.

A second aspect combinable with any of the previous aspects furtherincludes flowing the liquid phase through a liquid header from an outletof the condenser module and to a respective liquid conduit in each ofthe plurality of transport members.

A third aspect combinable with any of the previous aspects furtherincludes flowing the mixed phase through a vapor header to an inlet ofthe condenser module from a respective vapor conduit in each of theplurality of transport members.

A fourth aspect combinable with any of the previous aspects furtherincludes flowing the liquid phase of the working fluid through arespective fluid disconnect that couples each liquid conduit to theliquid header.

A fifth aspect combinable with any of the previous aspects furtherincludes operating the respective fluid disconnect to fluidly decouplethe liquid conduit from the liquid header and seal the liquid phase ofthe working fluid in at least one of the respective evaporator module orthe respective liquid conduit.

A sixth aspect combinable with any of the previous aspects furtherincludes circulating a flow of cooling air over the condenser module tochange the mixed phase of the working fluid to the liquid phase of theworking fluid.

A seventh aspect combinable with any of the previous aspects furtherincludes circulating a flow of cooling liquid through a cooling coil ofthe condenser module to change the mixed phase of the working fluid tothe liquid phase of the working fluid.

In an eighth aspect combinable with any of the previous aspects, thecondenser module includes a single condenser heat transfer coil.

A ninth aspect combinable with any of the previous aspects furtherincludes adjusting a position of at least one of a respective trayassembly that supports one or more heat-generating devices and aparticular evaporator module associated with the respective trayassembly; and based on the adjusting, urging the one or moreheat-generating devices into thermal contact with the particularevaporator module.

In another example implementation, a server rack thermosiphon systemincludes a plurality of evaporators, each evaporator including a thermalinterface for one or more heat-generating server rack devices; at leastone condenser mounted to an external structure of a server rack, thecondenser including a fluid-cooled heat transfer module; a liquidconduit that fluidly couples each of the evaporators to the condenser todeliver a liquid phase of a working fluid from the condenser to theevaporators; and a vapor conduit that fluidly couples each of theevaporators to the condenser to deliver a mixed phase of the workingfluid from the evaporators to the condenser.

In a first aspect combinable with the example implementation, a ratio ofthe at least one condenser to the plurality of evaporators is less thanone.

A second aspect combinable with any of the previous aspects furtherincludes a plurality of fluid disconnects that directly couple theliquid conduit to the evaporators.

In a third aspect combinable with any of the previous aspects, eachfluid disconnect is configured to decouple one of the evaporators fromthe condenser and seal the liquid phase of the working fluid in at leastone of the decoupled evaporator or the liquid conduit.

In a fourth aspect combinable with any of the previous aspects, the atleast one condenser includes an air-cooled condenser that includes aheat transfer coil and a fan.

In a fifth aspect combinable with any of the previous aspects, theplurality of evaporators include a plurality of fluid channels that eachinclude a liquid phase inlet and a liquid phase outlet.

A sixth aspect combinable with any of the previous aspects furtherincludes a plurality of server shells that at least partially enclose aplurality of server boards that support the one or more heat-generatingserver rack devices, the server shells defining the plurality of fluidchannels therebetween.

A seventh aspect combinable with any of the previous aspects furtherincludes at least one server board adjustment assembly positioned tourge one or more server boards into thermal contact with an interiorsurface of a server shell.

Various implementations of a data center cooling system according to thepresent disclosure may include one, some, or all of the followingfeatures. For example, the data center cooling system includes a serverrack scaled thermosiphon system that cools heat-generating electronicdevices (e.g., processors, network devices, memory modules, andotherwise) in a server rack without pumping or compression equipment,thereby requiring less input power. As another example, the thermosiphonsystem may reduce a cooling system power requirements of a dense rack,which may result in an improved power usage effectiveness (PUE) of adata center. As another example, a thermosiphon system that uses atwo-phase (e.g., liquid and vapor/mixed phase) cooling system maymaintain or help maintain the components at a tight temperature rangedue to the thermodynamic properties of the refrigerant. As yet anotherexample, the thermosiphon system may more efficiently expel heat fromthe server rack to an ambient workspace to minimize cooling losses.Further, the thermosiphon system may require less maintenance andexhibit reliability advantages as compared to other cooling systems,such as immersion systems. Further, the thermosiphon system may includea closed loop cooling fluid, thereby providing for removal andmaintenance of particular server boards without disruption of coolingother servers in the rack. As yet a further example, the thermosiphonsystem may be largely self-contained in a server rack footprint, therebyminimizing floor space usage. The thermosiphon system may also betailored to a particular cooling power criteria, e.g., by selecting aparticular refrigerant as a working fluid. Further, a closed loop systemof the thermosiphon system may physically separate the electronicequipment from the cooling fluid (e.g., refrigerant), thereby allowingstandard electronic components to be employed. As another example, thethermosiphon system may cool high power heat-generating components(e.g., processors and otherwise) with a passive, pump-less system thatuses less energy than forced circulation cooling systems. Thethermosiphon system may also provide higher cooling capability through atwo-phase boiling of a working fluid to cool the heat-generating devicesas compared to single-phase cooling systems (e.g., systems in which theworking fluid does not change phase to cool the devices).

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a schematic view of a server rack used in a datacenter environment that includes an example implementation of athermosiphon cooling system.

FIG. 2 illustrates a schematic view of a server rack used in a datacenter environment that includes another example implementation of athermosiphon cooling system.

FIG. 3 illustrates a schematic view of a server rack used in a datacenter environment that includes another example implementation of athermosiphon cooling system.

FIGS. 4A-4B illustrate schematic views of an example implementation of aserver board adjustment system used in a thermosiphon cooling system fora server rack.

FIG. 5 illustrates a schematic view of a server rack used in a datacenter environment that includes another example implementation of athermosiphon cooling system.

DETAILED DESCRIPTION

FIG. 1 illustrates an example system 100 that includes a server rack105, e.g., a 13 inch or 19 inch server rack, and multiple server racksub-assemblies 110 mounted within the rack 105. Although a single serverrack 105 is illustrated, server rack 105 may be one of a number ofserver racks within the system 100, which may include a server farm or aco-location facility that contains various rack mounted computersystems. Also, although multiple server rack sub-assemblies 110 areillustrated as mounted within the rack 105, there might be only a singleserver rack sub-assembly. Generally, the server rack 105 definesmultiple slots 107 that are arranged in an orderly and repeating fashionwithin the server rack 105, and each slot 107 is a space in the rackinto which a corresponding server rack sub-assembly 110 can be placedand removed. For example, the server rack sub-assembly can be supportedon rails 112 that project from opposite sides of the rack 105, and whichcan define the position of the slots 107.

The slots 107, and the server rack sub-assemblies 110, can be orientedwith the illustrated horizontal arrangement (with respect to gravity).Alternatively, the slots 107, and the server rack sub-assemblies 110,can be oriented vertically (with respect to gravity), although thiswould require some reconfiguration of the evaporator and condenserstructures described below. Where the slots are oriented horizontally,they may be stacked vertically in the rack 105, and where the slots areoriented vertically, they may be stacked horizontally in the rack 105.

Server rack 105, as part of a larger data center for instance, mayprovide data processing and storage capacity. In operation, a datacenter may be connected to a network, and may receive and respond tovarious requests from the network to retrieve, process, and/or storedata. In operation, for example, the server rack 105 typicallyfacilitates the communication of information over a network with userinterfaces generated by web browser applications of users who requestservices provided by applications running on computers in thedatacenter. For example, the server rack 105 may provide or help providea user who is using a web browser to access web sites on the Internet orthe World Wide Web.

The server rack sub-assembly 110 may be one of a variety of structuresthat can be mounted in a server rack. For example, in someimplementations, the server rack sub-assembly 110 may be a “tray” ortray assembly that can be slidably inserted into the server rack 105.The term “tray” is not limited to any particular arrangement, butinstead applies to motherboard or other relatively flat structuresappurtenant to a motherboard for supporting the motherboard in positionin a rack structure. In some implementations, the server racksub-assembly 110 may be a server chassis, or server container (e.g.,server box). In some implementations, the server rack sub-assembly 110may be a hard drive cage.

As illustrated in FIG. 1, the server rack sub-assembly 110 includes aframe or cage 120, a printed circuit board 122, e.g., a server ormotherboard, supported on the frame 120, one or more heat-generatingelectronic devices 124, e.g., a processor or memory or networkingdevice, mounted on the printed circuit board 122. Although the frame120, server board 122, and heat-generating electronic devices 124 areonly shown on one server rack sub-assembly 110, most or each of theassemblies 110 may include such components. Further, while notspecifically shown, the server board 122 may also support othercomponents, such as switch gear, one or more fans, and otherwise.

The frame 120 can include or simply be a flat structure onto which themotherboard 122 can be placed and mounted, so that the frame 120 can begrasped by technicians for moving the motherboard into place and holdingit in position within the rack 105. For example, the server racksub-assembly 110 may be mounted horizontally in the server rack 105 suchas by sliding the frame 120 into the slot 107 and over a pair of railsin the rack 105 on opposed sides of the server rack sub-assembly110—much like sliding a lunch tray into a cafeteria rack. The frame 120can have other forms (e.g., by implementing it as a peripheral framearound the motherboard) or may be eliminated so that the motherboarditself is located in, e.g., slidably engages, the rack 105. In addition,the frame 120 can include a flat plate and one or more side walls thatproject upwardly from the edges of the flat plate, and the flat platecould be the floor of a closed-top or open-top box or cage.

The illustrated server rack sub-assembly 110 includes a printed circuitboard 122, e.g., a motherboard, on which a variety of components aremounted, including heat-generating electronic devices 124. Although onemotherboard 122 is illustrated as mounted on the frame 120, multiplemotherboards may be mounted on the frame 120, depending on the needs ofthe particular application. In some implementations, one or more fanscan be placed on the frame 120 so that air enters at the front edge ofthe server rack sub-assembly 110, closer to the front of the rack 105when the sub-assembly 110 is installed in the rack 105, flows over themotherboard 122, over some of the heat-generating components on themotherboard 122, and is exhausted from the server rack assembly 110 atthe back edge, closer to the back of the rack 105 when the sub-assembly110 is installed in the rack 105. The one or more fans can be secured tothe frame 120 by brackets. Thus, the fans can pull air from within theframe 120 area and push the air after it has been warmed out of the rack105. An underside of the motherboard 122 can be separated from the frame120 by a gap.

FIG. 1 illustrates an example implementation of a thermosiphon coolingsystem that includes multiple evaporators 123 (one evaporator 123 isshown for simplicity) that are fluidly coupled to a condenser 115. Thethermosiphon system includes a liquid conduit 119 that fluidly couplesthe condenser 115 to each evaporator 123 so that a liquid phase 107 of aworking fluid (e.g., refrigerant or otherwise) is circulated (e.g.,through natural or forced circulation) from the condenser 115 to theevaporators 123. Each evaporator 123 is connected to the liquid conduit119 with a liquid connector 130.

The thermosiphon system also includes a vapor conduit 117 that fluidlycouples the condenser 115 to each evaporator 123 so that a mixed orvapor phase 103 of the working fluid is circulated (e.g., throughnatural or forced circulation) from the evaporators 123 to the condenser115. Each evaporator 123 is connected to the vapor conduit 117 with avapor connector 125.

In this example implementation, natural circulation is employed to movethe liquid phase 107 of the working fluid from the condenser 115 to theevaporators 123, and to move the vapor phase 103 of the working fluidfrom the evaporators 123 to the condenser 115. Thus, the illustratedimplementation of system 100 includes no pumps to circulate the workingfluid and, further, includes no compressors to implement a mechanicalrefrigeration vapor compression cycle to cool the working fluid. Inalternative implementations, a pump may be positioned in fluidcommunication with the liquid conduit 119 to forcibly circulate theliquid phase 107 of the working fluid from the condenser 115 to theevaporators 123.

Further, although the liquid conduit 119 and the vapor conduit 117 areillustrated in FIG. 1 as separate conduits, in alternativeimplementations, a single transport member may fluidly connect thecondenser 115 and the evaporators 123. The transport member may includeboth a liquid pathway and a vapor pathway, with the liquid pathwayconnected to the evaporators 123 by the liquid connectors 130, and thevapor pathway connected to the evaporators 123 by the vapor connectors125. Thus, although a single transport member may be used, the liquidphase 107 and the vapor (or mixed) phase 103 of the working fluid arestill separated during circulation between the condenser 115 and theevaporators 123.

In this illustrated example, the condenser 115 is positioned external tothe server rack 105. For example, the condenser 115 can be mounted ontop of the server rack 105, onto a side of the server rack 105, orotherwise positioned to receive a cooling fluid supply 140 and expel acooling fluid return 145. The cooling fluid may be air or liquid. Forexample, the cooling fluid supply 140 may be ambient air circulated(e.g., by a fan, not shown) through or across the condenser 115.

Although a single condenser 115 is illustrated in this example, theremay be multiple condensers 115. In some aspects, however, there may be aratio of condensers 115 to evaporators 123 that is less than one. Inother words, there may be more evaporators 123 than condensers 115, witheach condenser 115 fluidly coupled to multiple evaporators 123. Forinstance, in an example implementation, there may be two condensers 115to serve the evaporators 123 for the server rack 105. Thus, eachcondenser 115 would receive the vapor (or mixed) phase 103 from half(exactly or approximately) of the evaporators 123 and return the liquidphase 103 to the half of the evaporators 123. Other configurations ofcondensers 115 and evaporators 123 are within the scope of the presentdisclosure.

In an example operation, each evaporator 123 contacts one or moreelectronic devices 124 so that heat is drawn by conductive heat transferfrom the electronic device 124 to the evaporator 123. For example, theevaporator 123 is in conductive thermal contact with the electronicdevice 124. In particular, the bottom of the evaporator 123 contacts thetop of the electronic device 124. In operation, heat from the electronicdevice 124 causes the liquid phase 107 of the working fluid in theevaporator 123 to evaporate or change phase from liquid to a mixed phase(e.g., mixed liquid and vapor) or a vapor. The vapor (or mixed) phase103 then passes through the vapor conduit 117 to the condenser 115. Heatis transferred away from the condenser 115 into the cooling fluid supply140. The cooling fluid supply 140 may be air or liquid (e.g., chilled orcooled water or glycol, or otherwise). The cooling fluid return 145 mayexit the condenser 115 carrying the heat transferred from the electronicdevices 124. As heat is transferred away from the vapor (or mixed) phase103 in the condenser 115, a phase change occurs to change the vaporphase 103 back to the liquid phase 107.

FIG. 2 illustrates a schematic view of a server rack used in a datacenter environment that includes another example implementation of athermosiphon cooling system 200. The example thermosiphon cooling system200, as illustrated, includes a condenser 215 that is fluidly coupled tomultiple evaporators 227 that are thermally coupled to one or moreelectronic heat-generating devices 221 in a server rack 205. Forinstance, in this example implementation, a single air-cooled condenser215 may be fluidly coupled to the evaporators 227 or, in some aspects, aless than 1-to-1 ratio of condensers 215 may be fluidly coupled to theevaporators 227.

In this illustration, the server rack 205 is shown, e.g., in side view,and includes multiple server rack sub-assemblies 210 mounted within therack 205. Although a single server rack 205 is illustrated, server rack205 may be one of a number of server racks within the system 200, whichmay include a server farm or a co-location facility that containsvarious rack mounted computer systems. Also, although multiple serverrack sub-assemblies 210 are illustrated as mounted within the rack 205,there might be only a single server rack sub-assembly. Generally, theserver rack 205 defines multiple slots that are arranged in an orderlyand repeating fashion within the server rack 205, and each slot is aspace in the rack into which a corresponding server rack sub-assembly210 can be placed and removed. The slots, and the server racksub-assemblies 210, can be oriented with the illustrated horizontalarrangement (with respect to gravity) in this example. Where the slotsare oriented horizontally, they may be stacked vertically in the rack205, and where the slots are oriented vertically, they may be stackedhorizontally in the rack 205.

Server rack 205, as part of a larger data center for instance, mayprovide data processing and storage capacity. In operation, a datacenter may be connected to a network, and may receive and respond tovarious requests from the network to retrieve, process, and/or storedata. In operation, for example, the server rack 205 typicallyfacilitates the communication of information over a network with userinterfaces generated by web browser applications of users who requestservices provided by applications running on computers in thedatacenter. For example, the server rack 205 may provide or help providea user who is using a web browser to access web sites on the Internet orthe World Wide Web.

The server rack sub-assembly 210 may be one of a variety of structuresthat can be mounted in a server rack. For example, in someimplementations, the server rack sub-assembly 210 may be a “tray” ortray assembly (e.g., similar or identical to sub-assembly 110) that canbe slidably inserted into the server rack 205. In some implementations,the server rack sub-assembly 210 may be a server chassis, or servercontainer (e.g., server box). In some implementations, the server racksub-assembly 210 may be a hard drive cage.

As illustrated, each server rack sub-assembly 210 includes a printedcircuit board 224, e.g., a motherboard, on which a variety of componentsare mounted, including heat-generating electronic devices 221. Althoughone motherboard 224 is illustrated as mounted to each sub-assembly 210,multiple motherboards may be mounted in each sub-assembly 210, dependingon the needs of the particular application. In some implementations, oneor more fans can be coupled to the server board 224 so that air entersat one edge of the server rack sub-assembly 210 (e.g., closer to a frontof the rack 210 when the sub-assembly 210 is installed in the rack 205),flows over the motherboard 224, over some of the heat-generatingcomponents 221 on the motherboard 224, and is exhausted from the serverrack assembly 210 at another edge (e.g., a back edge closer to the backof the rack 205 when the sub-assembly 210 is installed in the rack 205).Thus, the fans can pull air from within the tray subassembly area andpush the air after it has been warmed out of the rack 205.

The illustrated implementation of the thermosiphon cooling system 200includes multiple evaporators 227 (e.g., one or more evaporators 227 perserver rack sub-assembly 210) that are fluidly coupled to the condenser215. The thermosiphon system 200 includes a liquid conduit 219 thatfluidly couples the condenser 215 to each evaporator 227 so that aliquid phase 207 of a working fluid (e.g., refrigerant or otherwise) iscirculated (e.g., through natural or forced circulation) from thecondenser 215 to the evaporators 227. Each evaporator 227 is connectedto the liquid conduit 219 with a liquid connector 230.

The thermosiphon system 200 also includes a vapor conduit 217 thatfluidly couples the condenser 215 to each evaporator 227 so that a mixedor vapor phase 203 of the working fluid is circulated (e.g., throughnatural or forced circulation) from the evaporators 227 to the condenser215. Each evaporator 227 is connected to the vapor conduit 217 with avapor connector 225.

One or more of the connectors (e.g., the liquid connectors 230, thevapor connectors 225, or both) may include a shut-off valve or devicethat fluidly isolates the respective evaporator 227 from the condenser215. For instance, the liquid connectors 230 (or vapor connectors 225 orboth) may be adjustable between an open and closed position. In the openposition, the respective evaporator 227 is fluidly connected to receivethe liquid phase 207 of the working fluid from the condenser 215. In theclosed position, the respective evaporator 227 is fluidly decoupled fromthe condenser 215 such that the liquid phase 207 of the working fluiddoes not flow from the condenser 215 to the evaporator 227 (orevaporators 227). In some aspects, in the closed position, the liquidconnector 225 may isolate and seal the liquid phase 207 in theevaporator 227 (or evaporators 227), thereby allowing removal of theserver rack sub-assembly 210 along with the evaporator 227 (orevaporators 227) as a single unit from the server rack 205. Inalternative aspects, in the closed position, the liquid connector 225may isolate and seal the liquid phase 207 in the condenser 215, therebyallowing removal of the server rack sub-assembly 210 along with theevaporator 227 (or evaporators 227) as a single unit from the serverrack 205.

In some aspects, one or more of the connectors (e.g., the liquidconnectors 230, the vapor connectors 225, or both) may also be used totailor or control an amount of flow of the liquid phase 207 to theevaporators 227. For example, in some aspects, one or more of the liquidconnectors 230 may also (e.g., in addition to a coupling/isolationdevice) act as an orifice that meters an amount of the liquid phase 207to the evaporator 227. In some examples, the connector 230 may be afixed orifice that allows a particular flow rate (e.g., maximum flowrate) of the liquid phase 207 to enter the evaporator 227. In otherexamples, the connector 230 may include or be a variable orifice that isadjustable, e.g., based on an amount of heat output from one or moreheat-generating devices 221 thermally coupled to the evaporator 227. Forexample, as the heat output of the one or more heat-generating devices221 increases, the variable orifice may allow an increased flow ofliquid phase 207 into the evaporator 227. As the heat output of the oneor more heat-generating devices 221 decreases, the variable orifice mayrestrict the flow of liquid phase 207 into the evaporator 227. Thus, theliquid connector 230 may be used to better match an amount of liquidphase 207 provided to the evaporator 227 to an amount of heat outputfrom the one or more heat-generating devices 221.

In this example implementation, natural circulation is employed to movethe liquid phase 207 of the working fluid from the condenser 215 to theevaporators 227, and to move the vapor phase 203 of the working fluidfrom the evaporators 227 to the condenser 215. Thus, the illustratedimplementation of system 200 includes no pumps to circulate the workingfluid and, further, includes no compressors to implement a mechanicalrefrigeration vapor compression cycle to cool the working fluid. Inalternative implementations, a pump may be positioned in fluidcommunication with the liquid conduit 219 to forcibly circulate theliquid phase 207 of the working fluid from the condenser 215 to theevaporators 227.

Further, although the liquid conduit 219 and the vapor conduit 217 areillustrated in FIG. 2 as separate conduits, in alternativeimplementations, a single transport member may fluidly connect thecondenser 215 and the evaporators 227. The transport member may includeboth a liquid pathway and a vapor pathway, with the liquid pathwayconnected to the evaporators 227 by the liquid connectors 230, and thevapor pathway connected to the evaporators 227, through vapor stubs 227,by the vapor connectors 225. Thus, although a single transport membermay be used, the liquid phase 207 and the vapor (or mixed) phase 203 ofthe working fluid are still separated during circulation between thecondenser 215 and the evaporators 227.

In this illustrated example, the condenser 215 is positioned external tothe server rack 205. For example, the condenser 215 can be mounted ontop of the server rack 205, onto a side of the server rack 205, orotherwise positioned to receive a cooling fluid supply 240 and expel acooling fluid return 245. In this example implementation, the condenser215 is an air-cooled condenser 215, which includes a coil section 235and a fan section 237. The coil section 235, as shown, includes acondenser coil 239 through which the working fluid is circulated and oneor more heat transfer surfaces 241 (e.g., fins) thermally coupled to thecoil 239. The coil tube 239 receives the vapor (or mixed) phase 203 ofthe working fluid from the vapor conduit 217 and, supplies the liquidphase 207 of the working fluid to the liquid conduit 219. The coolingfluid supply 240 may be ambient air circulated through or across thecondenser coil 239.

Although a single condenser 215 is illustrated in this example, theremay be multiple condensers 215. In some aspects, however, there may be aratio of condensers 215 to evaporators 227 that is less than one. Inother words, there may be more evaporators 227 than condensers 215, witheach condenser 215 fluidly coupled to multiple evaporators 227. Forinstance, in an example implementation, there may be two condensers 215to serve the evaporators 227 for the server rack 205. Thus, eachcondenser 215 would receive the vapor (or mixed) phase 203 from half(exactly or approximately) of the evaporators 227 and return the liquidphase 203 to the half of the evaporators 227.

This may allow, in some instances, for redundant cooling of theheat-generating devices 221. For example, in some aspects, there may bemultiple evaporators 227 per server rack sub-assembly 210. Eachevaporator 227 positioned for the server rack sub-assembly 210 may be inthermal contact (e.g., through a thermal interface or thermal interfacematerial) with a particular heat-generating device 221 (e.g., processoror otherwise) or multiple heat-generating devices 221. Thus, differentheat-generating devices 221 may be in thermal contact with differentones of the multiple evaporators 227 for the particular server racksub-assembly 210. Also, each heat-generating device 221 on a server racksub-assembly 210 may be in thermal contact with multiple evaporators227. A failure in one particular evaporator 227 of a server racksub-assembly 210 may not, therefore, remove cooling for the entireserver rack sub-assembly 210 or even a single heat-generating device221. Further, in aspects in which evaporators 227 of a single serverrack sub-assembly 210 are fluidly coupled to different condensers 215 ofa system 200 that includes multiple condenser 215, a failure of a singlecondenser 215 may not remove cooling for the entire server racksub-assembly 210 or even a single heat-generating device 221.

In an example operation, each evaporator 227 contacts one or moreelectronic devices 221 so that heat is drawn by conductive heat transferfrom the electronic device 221 to the evaporator 227. For example, theevaporator 227 is in conductive thermal contact with the electronicdevice 221 (or in thermal contact with the device 221 through a thermalinterface material). In particular, the bottom of the evaporator 227contacts the top of the electronic device 221. In operation, heat fromthe electronic device 221 causes the liquid phase 207 of the workingfluid in the evaporator 227 to evaporate or change phase from liquid toa mixed phase (e.g., mixed liquid and vapor) or a vapor. The vapor (ormixed) phase 203 then passes through the vapor stub 223 to the vaporconduit 217 and to the condenser 215. Heat is transferred away from thecondenser 215 into the cooling fluid supply 240. For example, thecooling fluid supply 240 may be drawn or circulated by the fan 237 overthe condenser coil 239 in order to remove heat from the vapor phase 203to the cooling fluid supply 240. The cooling fluid return 245 may exitthe condenser 215 carrying the heat transferred from the electronicdevices 221. As heat is transferred away from the vapor (or mixed) phase203 in the condenser 215, a phase change occurs to change the vaporphase 203 back to the liquid phase 207. The liquid phase 207 iscirculated (e.g., naturally or otherwise) to the liquid conduit 219 tobe provided to the evaporators 227 to repeat the process.

FIG. 3 illustrates a schematic view of a server rack used in a datacenter environment that includes another example implementation of athermosiphon cooling system 300. The example thermosiphon cooling system300, as illustrated, includes a condenser 315 that is fluidly coupled tomultiple evaporators 327 that are thermally coupled to one or moreelectronic heat-generating devices 321 in a server rack 305. Forinstance, in this example implementation, a single condenser 315 may befluidly coupled to the evaporators 327 or, in some aspects, a less than1-to-1 ratio of condensers 315 may be fluidly coupled to the evaporators327. In this illustrated example, the thermosiphon system 300 may beused to cool heat-generating devices on vertically-mounted server racksub-assemblies in the server rack 305.

In this illustration, the server rack 305 is shown, e.g., in side view,and includes multiple server rack sub-assemblies 310 mounted verticallywithin the rack 305. Although a single server rack 305 is illustrated,server rack 305 may be one of a number of server racks within the system300, which may include a server farm or a co-location facility thatcontains various rack mounted computer systems. Also, although multipleserver rack sub-assemblies 310 are illustrated as mounted within therack 305, there might be only a single server rack sub-assembly.Generally, the server rack 305 defines multiple slots that are arrangedin an orderly and repeating fashion within the server rack 305, and eachslot is a space in the rack into which a corresponding server racksub-assembly 310 can be placed and removed. The slots, and the serverrack sub-assemblies 310, are oriented in a vertical arrangement (withrespect to gravity) in this example.

Server rack 305, as part of a larger data center for instance, mayprovide data processing and storage capacity. In operation, a datacenter may be connected to a network, and may receive and respond tovarious requests from the network to retrieve, process, and/or storedata. In operation, for example, the server rack 305 typicallyfacilitates the communication of information over a network with userinterfaces generated by web browser applications of users who requestservices provided by applications running on computers in thedatacenter. For example, the server rack 305 may provide or help providea user who is using a web browser to access web sites on the Internet orthe World Wide Web.

The server rack sub-assembly 310 may be one of a variety of structuresthat can be mounted in a server rack. For example, in someimplementations, the server rack sub-assembly 310 may be a “tray” ortray assembly (e.g., similar or identical to sub-assembly 110) that canbe slidably inserted into the server rack 305. In some implementations,the server rack sub-assembly 310 may be a server chassis, or servercontainer (e.g., server box). In some implementations, the server racksub-assembly 310 may be a hard drive cage.

As illustrated, each server rack sub-assembly 310 includes a printedcircuit board 324, e.g., a motherboard, on which a variety of componentsare mounted, including heat-generating electronic devices 321. One ormore motherboards 324 are mounted to each sub-assembly 310, multiplemotherboards may be mounted in each sub-assembly 310, depending on theneeds of the particular application. In some implementations, one ormore fans can be coupled to the server board 324 so that air enters atone edge of the server rack sub-assembly 310 (e.g., closer to a front ofthe rack 310 when the sub-assembly 310 is installed in the rack 305),flows over the motherboard 324, over some of the heat-generatingcomponents 321 on the motherboard 324, and is exhausted from the serverrack assembly 310 at another edge (e.g., a back edge closer to the backof the rack 305 when the sub-assembly 310 is installed in the rack 305).Thus, the fans can pull air from within the tray subassembly area andpush the air after it has been warmed out of the rack 305.

The illustrated implementation of the thermosiphon cooling system 300includes multiple evaporators 327 (e.g., one or more evaporators 327 perserver rack sub-assembly 310) that are fluidly coupled to the condenser315. The thermosiphon system 300 includes a liquid conduit 319 thatfluidly couples the condenser 315 to each evaporator 327 so that aliquid phase 307 of a working fluid (e.g., refrigerant or otherwise) iscirculated (e.g., through natural or forced circulation) from thecondenser 315 to the evaporators 327. Each evaporator 327 is connectedto the liquid conduit 319 with a liquid connector 330. In this example,implementation, the liquid conduit 319 is positioned at or near a bottomof the server rack 305 so that the liquid phase 307 of the working fluidcan flow (e.g., by gravity) from the condenser 315 through the conduit319 to enter a bottom of the evaporators 327.

The thermosiphon system 300 also includes a vapor conduit 317 thatfluidly couples the condenser 315 to each evaporator 327 so that a mixedor vapor phase 303 of the working fluid is circulated (e.g., throughnatural or forced circulation) from the evaporators 327 to the condenser315. In this example, each evaporator 327 is connected to the vaporconduit 317 with a vapor connector 325.

One or more of the connectors (e.g., the liquid connectors 330, thevapor connectors 325, or both) may include a shut-off valve or devicethat fluidly isolates the respective evaporator 327 from the condenser315. For instance, the liquid connectors 330 (or vapor connectors 325 orboth) may be adjustable between an open and closed position. In the openposition, the respective evaporator 327 is fluidly connected to receivethe liquid phase 307 of the working fluid from the condenser 315. In theclosed position, the respective evaporator 327 is fluidly decoupled fromthe condenser 315 such that the liquid phase 307 of the working fluiddoes not flow from the condenser 315 to the evaporator 327 (orevaporators 327). In some aspects, in the closed position, the liquidconnector 325 may isolate and seal the liquid phase 307 in theevaporator 327 (or evaporators 327), thereby allowing removal of theserver rack sub-assembly 310 along with the evaporator 327 (orevaporators 327) as a single unit from the server rack 305. Inalternative aspects, in the closed position, the liquid connector 325may isolate and seal the liquid phase 307 in the condenser 315, therebyallowing removal of the server rack sub-assembly 310 along with theevaporator 327 (or evaporators 327) as a single unit from the serverrack 305.

In some aspects, one or more of the connectors (e.g., the liquidconnectors 330, the vapor connectors 325, or both) may also be used totailor or control an amount of flow of the liquid phase 307 to theevaporators 327. For example, in some aspects, one or more of the liquidconnectors 330 may also (e.g., in addition to a coupling/isolationdevice) act as an orifice that meters an amount of the liquid phase 307to the evaporator 327. In some examples, the connector 330 may be afixed orifice that allows a particular flow rate (e.g., maximum flowrate) of the liquid phase 307 to enter the evaporator 327. In otherexamples, the connector 330 may include or be a variable orifice that isadjustable, e.g., based on an amount of heat output from one or moreheat-generating devices 321 thermally coupled to the evaporator 327. Forexample, as the heat output of the one or more heat-generating devices321 increases, the variable orifice may allow an increased flow ofliquid phase 307 into the evaporator 327. As the heat output of the oneor more heat-generating devices 321 decreases, the variable orifice mayrestrict the flow of liquid phase 307 into the evaporator 327. Thus, theliquid connector 330 may be used to better match an amount of liquidphase 307 provided to the evaporator 327 to an amount of heat outputfrom the one or more heat-generating devices 321.

In alternative aspects, one, some, or all of the connectors (e.g.,connectors 325 and connectors 330) may not be included in order forremoval of the server rack sub-assemblies 310 from the server rack 305.For example, a position of each server rack sub-assembly 310, or eachserver board 324, may be adjusted between an unengaged position in whichthe heat-generating devices 321 are not in thermal contact with theevaporator 327 (or evaporators 327) and an engaged position in which theheat-generating devices 321 are in thermal contact with the evaporator327 (or evaporators 327). Thus, in the unengaged position, the serverrack sub-assembly 310 or the server board 324 may be removed from therack 305 (e.g., for servicing or otherwise) without disturbing thethermosiphon cooling system 300.

For example, turning briefly to FIGS. 4A-4B, these figures illustrateschematic views of an example implementation of a server boardadjustment system 350 used in a thermosiphon cooling system for a serverrack. In this example, the server board adjustment system 350 may beused to adjust the server rack sub-assembly 310 or the server board 324between the unengaged position and the engaged position. In thisexample, FIG. 4A shows the server rack sub-assembly 310 or the serverboard 324 in the unengaged position in which there is a gap 393 betweenthe heat-generating device 321 (or a thermal interface material 358mounted on the heat-generating device 321) and the evaporator 327. Thethermal interface material 358, in some aspects, includes a pliablematerial (e.g., putty, semi-solid, gel, or otherwise) that increases aphysical contact area between the heat-generating device 321 and theevaporator 327 (e.g., due to imperfections in the solid surfaces ofthese components). Further, as heat is transferred through the thermalinterface material 358, the material 358 may undergo phase change (e.g.,from solid to semi-solid) to further increase the physical contact area(e.g., by flowing into or filling in small gaps between the solidsurfaces).

The example server board adjustment system 350 includes a cam 352coupled to the server rack 305 and positioned to adjust a frame 354 ofthe server rack sub-assembly 310. The frame 354 supports the serverboard 324 on members 356. In the unengaged position, the liquid phase307 circulates through the evaporator 327, as no or negligible heat fromthe heat-generating device 321 is transferred to the liquid phase 307.

FIG. 4B shows the server rack sub-assembly 310 or the server board 324in the engaged position in which there is thermal contact between theheat-generating device 321 (or the thermal interface material 358mounted on the heat-generating device 321) and the evaporator 327. Toadjust from the unengaged position to the engaged position, the cam 352is operated (e.g., by an actuator, motor, or servo controlled by acontroller or control system, not shown) to push the frame 354 of theserver rack sub-assembly 310 toward the evaporator 327. In pushing theframe 354 (and thus the server board 324 and the heat-generating device321) towards the evaporator 327, thermal contact between theheat-generating device 321 and the evaporator 327 is achieved totransfer heat to the liquid phase 307 (to boil the liquid phase 307 intobubbles 360 of the vapor phase 303). In some aspects, operation of thecam 352 may be initiated by a signal from a controller that signifies,for instance, that a temperature of the device 321 exceeds a threshold,the server rack sub-assembly 310 is in proper position, or otherwise.

Although FIGS. 4A-4B illustrate an example implementation of a serverboard adjustment system 350 (e.g., a cam system), other implementationsare contemplated by the present disclosure. For example, the serverboard adjustment system 350 may include a piston device that can beactuated to urge the heat-generating device 321 against the evaporator327. As another example, an expandable bladder may be actuated to urgethe heat-generating device 321 against the evaporator 327. As yetanother example, a spring-loaded cam or piston may be actuated to urgethe heat-generating device 321 against the evaporator 327.

Moreover, in some aspects, the server board adjustment system 350 mayurge the evaporator 327 into thermal engagement with the heat-generatingdevice 321 that remain stationary (exactly or substantially). In someother aspects, the server board adjustment system 350 may urge theevaporator 327 and the heat-generating device 321 into thermalengagement.

Returning to FIG. 3, in this example implementation, natural circulationis employed to move the liquid phase 307 of the working fluid from thecondenser 315 to the evaporators 327, and to move the vapor phase 303 ofthe working fluid from the evaporators 327 to the condenser 315. Thus,the illustrated implementation of system 300 includes no pumps tocirculate the working fluid and, further, includes no compressors toimplement a mechanical refrigeration vapor compression cycle to cool theworking fluid. In alternative implementations, a pump may be positionedin fluid communication with the liquid conduit 319 to forcibly circulatethe liquid phase 307 of the working fluid from the condenser 315 to theevaporators 327.

In this illustrated example, the condenser 315 is positioned external tothe server rack 305. For example, the condenser 315 can be mounted ontop of the server rack 305, onto a side of the server rack 305, orotherwise positioned to receive a cooling fluid supply 340 and expel acooling fluid return 345. In this example implementation, the condenser315 is an air-cooled or liquid-cooled condenser 315. Cooling fluidsupply 340 (e.g., cooled or ambient air, or chilled or cooled water orglycol) may be circulated through or across the condenser 315.

Although a single condenser 315 is illustrated in this example, theremay be multiple condensers 315. In some aspects, however, there may be aratio of condensers 315 to evaporators 327 that is less than one. Inother words, there may be more evaporators 327 than condensers 315, witheach condenser 315 fluidly coupled to multiple evaporators 327. Forinstance, in an example implementation, there may be two condensers 315to serve the evaporators 327 for the server rack 305. Thus, eachcondenser 315 would receive the vapor (or mixed) phase 303 from half(exactly or approximately) of the evaporators 327 and return the liquidphase 303 to the half of the evaporators 327.

This may allow, in some instances, for redundant cooling of theheat-generating devices 321. For example, in some aspects, there may bemultiple evaporators 327 per server rack sub-assembly 310. Eachevaporator 327 positioned for the server rack sub-assembly 310 may be inthermal contact (e.g., through a thermal interface or thermal interfacematerial) with a particular heat-generating device 321 (e.g., processoror otherwise) or multiple heat-generating devices 321. Thus, differentheat-generating devices 321 may be in thermal contact with differentones of the multiple evaporators 327 for the particular server racksub-assembly 310. Also, each heat-generating device 321 on a server racksub-assembly 310 may be in thermal contact with multiple evaporators327. A failure in one particular evaporator 327 of a server racksub-assembly 310 may not, therefore, remove cooling for the entireserver rack sub-assembly 310 or even a single heat-generating device321. Further, in aspects in which evaporators 327 of a single serverrack sub-assembly 310 are fluidly coupled to different condensers 315 ofa system 300 that includes multiple condenser 315, a failure of a singlecondenser 315 may not remove cooling for the entire server racksub-assembly 310 or even a single heat-generating device 321.

In an example operation, each evaporator 327 contacts one or moreelectronic devices 321 so that heat is drawn by conductive heat transferfrom the electronic device 321 to the evaporator 327. For example, theevaporator 327 may be or may be placed into conductive thermal contactwith the electronic device 321 (or in thermal contact with the device321 through a thermal interface material). In particular, the bottom ofthe evaporator 327 contacts the top of the electronic device 321. Inoperation, heat from the electronic device 321 causes the liquid phase307 of the working fluid in the evaporator 327 to evaporate or changephase from liquid to a mixed phase (e.g., mixed liquid and vapor) or avapor. The vapor (or mixed) phase 303 then passes through the vapor stub323 to the vapor conduit 317 and to the condenser 315. Heat istransferred away from the condenser 315 into the cooling fluid supply340. For example, the cooling fluid supply 340 may be drawn orcirculated through or into the condenser 315 in order to remove heatfrom the vapor phase 303 to the cooling fluid supply 340. The coolingfluid return 345 may exit the condenser 315 carrying the heattransferred from the electronic devices 321. As heat is transferred awayfrom the vapor (or mixed) phase 303 in the condenser 315, a phase changeoccurs to change the vapor phase 303 back to the liquid phase 307. Theliquid phase 307 is circulated (e.g., naturally or otherwise) to theliquid conduit 319 to be provided to the evaporators 327 to repeat theprocess.

FIG. 5 illustrates a schematic view of a server rack 505 used in a datacenter environment that includes another example implementation of athermosiphon cooling system 500. The example thermosiphon cooling system500, as illustrated, includes a condenser 515 that is fluidly coupled tomultiple liquid channels that form evaporator sections 550 that arethermally coupled to one or more electronic heat-generating devices 521in a server rack 505. For instance, in this example implementation, asingle air-cooled condenser 515 may be fluidly coupled to theevaporators 550 or, in some aspects, a less than 1-to-1 ratio ofcondensers 515 may be fluidly coupled to the evaporators 550. In thisexample system 500, a working fluid used to cool the electronicheat-generating devices 521 is contained in the server rack 505 butotherwise may be free (e.g., not contained in an enclosed conduit) tomigrate through the rack 505, for example, depending on heat beingtransferred to and from the working fluid. In this example, system, theserver boards 524 to which the heat-generating devices 521 are mountedare enclosed (e.g., all or substantially) in respective server shells560 (e.g., thermally conductive shells). Together, each shell 560 thatencloses one or more server boards 524 (as well as multipleheat-generating devices 521) may form a server rack sub-assembly 510. Inthis example system 500, a liquid-cooled condenser 515 (or condensers515) may operate to change a vapor (or mixed) phase 503 of the workingfluid to a liquid phase 507 of the working fluid.

In this illustration, the server rack 505 is shown, e.g., in side view,and includes multiple server rack sub-assemblies 510 mounted within therack 505. Although a single server rack 505 is illustrated, server rack505 may be one of a number of server racks within the system 500, whichmay include a server farm or a co-location facility that containsvarious rack mounted computer systems. Also, although multiple serverrack sub-assemblies 510 are illustrated as mounted within the rack 505,there might be only a single server rack sub-assembly. Generally, theserver rack 505 defines multiple slots that are arranged in an orderlyand repeating fashion within the server rack 505, and each slot is aspace in the rack into which a corresponding server rack sub-assembly510 can be placed and removed. The slots, and the server racksub-assemblies 510, can be oriented with the illustrated verticalarrangement (with respect to gravity) in this example. Where the slotsare oriented vertically, they may be stacked horizontally in the rack505 in alternative implementations.

Server rack 505, as part of a larger data center for instance, mayprovide data processing and storage capacity. In operation, a datacenter may be connected to a network, and may receive and respond tovarious requests from the network to retrieve, process, and/or storedata. In operation, for example, the server rack 505 typicallyfacilitates the communication of information over a network with userinterfaces generated by web browser applications of users who requestservices provided by applications running on computers in thedatacenter. For example, the server rack 505 may provide or help providea user who is using a web browser to access web sites on the Internet orthe World Wide Web.

The server rack sub-assembly 510 may be one of a variety of structuresthat can be mounted in a server rack. For example, in someimplementations, the server rack sub-assembly 510 may be a “tray” ortray assembly (e.g., similar or identical to sub-assembly 110) that canbe slidably inserted into the server rack 505. In some implementations,the server rack sub-assembly 510 may be a server chassis, or servercontainer (e.g., server box). In some implementations, the server racksub-assembly 510 may be a hard drive cage.

As illustrated, each server rack sub-assembly 510 includes a printedcircuit board 524, e.g., a motherboard, on which a variety of componentsare mounted, including heat-generating electronic devices 521. Althoughone motherboard 524 is illustrated as mounted to each sub-assembly 510,multiple motherboards may be mounted in each sub-assembly 510, dependingon the needs of the particular application. In some implementations,each sub-assembly 510 includes a server board adjustment system asdescribed with reference to FIGS. 4A-4B. In system 500, a server boardadjustment system may be used to adjust the server board 524 between anunengaged position and an engaged position. In the unengaged position,there may be a gap between the heat-generating device(s) 521 (or athermal interface material mounted on the heat-generating device) andthe server shell 560, which is in thermal contact with one or moreevaporators 550. In the unengaged position, the liquid phase 507circulates through the evaporator 550 but is not in thermal contact withthe heat-generating devices 521, so no or negligible heat from theheat-generating device 521 is transferred to the liquid phase 507. Inthe engaged position, thermal contact between the heat-generating device521 (or a thermal interface material mounted on the heat-generatingdevice 521) and the evaporator 550 through the server shell 560 iscreated by the server board adjustment system. For example, the serverboard adjustment system may adjust (e.g., push) the server board 524 sothat the heat-generating devices 521 come into thermal contact with aninner surface of the server shell 560. Heat from the devices 521 maythus be transferred through the server shell 560 to the evaporator 550.

The illustrated implementation of the thermosiphon cooling system 500includes multiple evaporators 550 that are fluidly coupled to thecondenser 515. In some aspects, each evaporator 550 may be a fluidchannel formed between adjacent server shells 560, e.g., without aseparate conduit to enclose the liquid phase 507 of the working fluid inthe channels. In some aspects, each evaporator 550 may include a liquidconduit that encloses the liquid phase 507 of the working fluid betweenadjacent server shells 560.

In this example implementation of system 500, natural circulation isemployed to move the liquid phase 507 of the working fluid from thecondenser 515 to the evaporators 550, and to move the vapor phase 503 ofthe working fluid from the evaporators 550 to the condenser 515. Thus,the illustrated implementation of system 500 includes no pumps tocirculate the working fluid and, further, includes no compressors toimplement a mechanical refrigeration vapor compression cycle to cool theworking fluid. In alternative implementations, a pump may be positionedin the server rack 505 to circulate liquid phase 507 of the workingfluid from the condenser 515 to a bottom portion of the rack 505 toenter the evaporators 550.

In this illustrated example, the condenser 515 is positioned internal tothe server rack 505 at or near a top portion of the rack 505. In thisexample implementation, the condenser 515 is a liquid-cooled condenser515, which includes a condenser coil 539 over which the working fluid iscirculated and one or more heat transfer surfaces 541 (e.g., fins)thermally coupled to the coil 539. The coil 539 receives the vapor (ormixed) phase 503 of the working fluid from the evaporators 550 (e.g.,from top outlets 551 of the evaporators 550) and supplies the liquidphase 507 of the working fluid back to bottom inlets 553 of theevaporators 550.

In this example, a cooling liquid supply 525 (e.g., chilled or cooledwater or glycol) may be circulated through the condenser coil 539. Acooling liquid return 530, including heat from the heat-generatingdevices 521, may be circulated from the coil 539 to a cooling liquidsource (e.g., chiller, cooling tower, direct expansion condensing unit,heat exchanger, or other cooling liquid source).

Although a single condenser coil 539 is illustrated in this example,there may be multiple condenser coils 539. In some aspects, however,there may be a ratio of condenser coils 539 to evaporators 550 that isless than one. In other words, there may be more evaporators 550 thancondenser coils 539, with each condenser coil 539 positioned to receivethe vapor (or mixed) phase 503 of the working fluid from multipleevaporators 550. For instance, in an example implementation, there maybe two condenser coils 539 to serve the evaporators 550 for the serverrack 505. Thus, each condenser coil 539 would receive the vapor (ormixed) phase 503 from half (exactly or approximately) of the evaporators550 and return the liquid phase 503 to the half of the evaporators 550.In some aspects, return of the liquid phase 503 to the bottom inlets 553may occur by gravity feed from the condenser coils 539 to the bottomportion of the server rack 505. Further, each condenser coil 539 mayhave a respective cooling liquid supply 525 and cooling liquid return530 through connections 535.

This may allow, in some instances, for redundant cooling of theheat-generating devices 521. For example, in some aspects, there may bemultiple evaporators 550 per server rack sub-assembly 510 (e.g.,multiple liquid conduits within each evaporator 550). Each evaporator550 positioned for the server rack sub-assembly 510 may be in thermalcontact (e.g., through a thermal interface or thermal interfacematerial) with a particular heat-generating device 521 (e.g., processoror otherwise) or multiple heat-generating devices 521. Thus, differentheat-generating devices 521 may be in thermal contact with differentones of the multiple evaporators 550 for the particular server racksub-assembly 510. Also, each heat-generating device 521 on a server racksub-assembly 510 may be in thermal contact with multiple evaporators 550(e.g., multiple liquid conduits within each evaporator 550). A failurein one particular evaporator 550 of a server rack sub-assembly 510 maynot, therefore, remove cooling for the entire server rack sub-assembly510 or even a single heat-generating device 521. Further, in aspects inwhich evaporators 550 of a single server rack sub-assembly 510 arefluidly coupled to different condenser coils 539 of a system 500 thatincludes multiple condenser coils 539, a failure of a single condensercoil 539 may not remove cooling for the entire server rack sub-assembly510 or even a single heat-generating device 521.

In an example operation, each evaporator 550 is placed into thermalcontact one or more electronic devices 521, for example, by adjustingeach server board 524 to thermally contact the particular server shell560 that encloses the board 524. Heat is drawn by conductive heattransfer from the electronic device 521 to the evaporator 550. Forexample, the evaporator 550 is in conductive thermal contact with theelectronic device 521 through the server shell 560 (or in thermalcontact with the device 521 through a thermal interface material on thedevice 521 or on the shell 560). In operation, heat from the electronicdevice 521 causes the liquid phase 507 of the working fluid in theevaporator 550 to evaporate or change phase from liquid to a mixed phase(e.g., mixed liquid and vapor) or a vapor. The vapor (or mixed) phase503 then circulates (e.g., naturally by density differences) upwardthrough the top outlets 551 to the condenser 515. Heat is transferredaway from the condenser 515 into the cooling liquid supply 525. Forexample, the cooling liquid supply 525 may be pumped through thecondenser coil 539 in order to remove heat from the vapor phase 503 tothe cooling liquid supply 525. Heat is transferred from the vapor phase503 as it flows around in thermal contact with the condenser coil 539and the fins 541. The cooling liquid return 530 circulates from the coil539 carrying the heat transferred from the electronic devices 521. Asheat is transferred away from the vapor (or mixed) phase 503 in thecondenser 515, a phase change occurs to change the vapor phase 503 backto the liquid phase 507. The liquid phase 507 is circulated (e.g.,naturally or otherwise) to a bottom portion of the server rack 505 to beprovided to the evaporators 550 to repeat the process.

A number of embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made without departing fromthe spirit and scope of what is described. For example, the steps of theexemplary operations described herein may be performed in other orders,some steps may be removed, and other steps may be added. Accordingly,other embodiments are within the scope of the following claims.

What is claimed is:
 1. A data center server rack system, comprising: aserver rack that supports a plurality of data center electronic devicesmounted on a plurality of server board assemblies within a volume of ahousing of the server rack; a plurality of evaporator modules positionedin the housing and in thermal communication with the plurality of datacenter electronic devices; a condenser that comprises a heat exchangerin fluid communication with the plurality of evaporator modules andmounted to the housing; a fluid coolant that circulates through theplurality of evaporator modules to receive heat from the plurality ofdata center electronic devices such that at least a portion of the fluidcoolant changes to a vapor phase that is circulated to the heatexchanger of the condenser to change into a liquid phase of the fluidcoolant; and a plurality of shells, each shell comprising a volume thatencloses a particular server board assembly of the plurality of serverboard assemblies and is fluidly isolated from the fluid coolant thatcirculates through at least one of the plurality of evaporator modulesby the shell, the shell further comprising an exterior surface that isin fluid contact with the fluid coolant.
 2. The system of claim 1,wherein each of the evaporator modules comprises a flow path through thevolume adjacent at least one of the server board assemblies, each flowpath in fluid communication with the other flow paths within the volume.3. The system of claim 1, wherein the fluid coolant naturally circulatesthrough the plurality of evaporator modules to receive heat from theplurality of data center electronic devices such that the portion of thefluid coolant changes to the vapor phase that is naturally circulated tothe heat exchanger of the condenser to change into the liquid phase ofthe fluid coolant.
 4. The system of claim 1, wherein the volumecomprises a vapor phase volume portion and a liquid phase volumeportion.
 5. The system of claim 1, further comprising a thermalinterface material mounted on at least one of the data center electronicdevices mounted on the particular server board assembly.
 6. The systemof claim 1, wherein the particular server board assembly comprises aspring-loaded cam or piston positioned to urge the particular serverboard assembly toward an interior surface of the shell to urge the datacenter electronic devices mounted on the particular server boardassembly into direct conductive thermal contact with the fluid coolantthrough the interior surface of the shell.
 7. The system of claim 6,wherein the shell is in at least one of conductive or convective thermalcontact with a particular one of the plurality of evaporator modules. 8.The system of claim 1, wherein the condenser comprises a singlecondenser.
 9. The system of claim 8, wherein the single condensercomprises a liquid cooled condenser having at least one condenser liquidinlet and at least one condenser liquid outlet.
 10. The system of claim8, wherein the single condenser is mounted within the housing of theserver rack.
 11. A method for cooling a plurality of data centerelectronic devices, comprising: circulating a liquid phase of a fluidcoolant through a plurality of evaporator modules that are positioned ina volume of a housing of a server rack that supports the plurality ofdata center electronic devices mounted on a plurality of server boardassemblies within the volume of the housing, a particular server boardassembly enclosed in a volume of a shell that comprises an exteriorsurface in fluid contact with the fluid coolant; fluidly isolating thedata center electronic devices mounted on the particular server boardfrom the fluid coolant by the shell; transferring heat from theplurality of data center electronic devices, through the shell, to theliquid phase of the fluid coolant circulating within the plurality ofevaporator modules; vaporizing, with the transferred heat, at least aportion of the liquid phase of the fluid coolant to a vapor phase of thefluid coolant; circulating the vapor phase of the fluid coolant from theplurality of evaporator modules to a condenser that comprises a heatexchanger mounted to the housing; and condensing, with the heatexchanger, the vapor phase of the fluid coolant to a liquid phase of thefluid coolant.
 12. The method of claim 11, wherein each of theevaporator modules comprises a flow path through the volume adjacent atleast one of the server board assemblies, each flow path in fluidcommunication with the other flow paths within the volume.
 13. Themethod of claim 11, wherein circulating the liquid phase of the fluidcoolant comprises naturally circulating the liquid phase of the fluidcoolant, and circulating the vapor phase of the fluid coolant comprisesnaturally circulating the vapor phase of the fluid coolant.
 14. Themethod of claim 13, wherein naturally circulating the liquid phase ofthe fluid coolant comprises naturally circulating the liquid phase froma vapor phase volume portion of the volume to a liquid phase volumeportion of the volume, and naturally circulating the vapor phase of thefluid coolant comprises naturally circulating the vapor phase from theliquid phase volume portion of the volume to the vapor phase volumeportion of the volume.
 15. The method of claim 11, wherein the condensercomprises a single condenser.
 16. The method of claim 15, furthercomprising removing heat from the single condenser with a flow of acondenser liquid.
 17. The method of claim 15, wherein the singlecondenser is mounted within the housing of the server rack.
 18. Themethod of claim 11, further comprising urging the particular serverboard assembly toward an interior surface of the shell to move the datacenter electronic devices mounted on the particular server boardassembly into direct conductive thermal contact with the liquid phase ofthe fluid coolant through the interior surface of the shell.
 19. Themethod of claim 18, further comprising: conductively transferring heatfrom the data center electronic devices mounted on the particular serverboard assembly to the shell; and at least one of conductively orconvectively transferring the heat from the shell into the fluid coolantcirculating in at least one of the plurality of evaporator modules. 20.The method of claim 19, further comprising conductively transferringheat through a thermal interface material mounted on at least one of thecenter electronic devices mounted on the particular server boardassembly to the shell.